The present invention relates to a method and an apparatus for adjusting the static convergence and/or purity of color television tubes, in particular of the kind comprising a shadow mask.
It is known that a color television tube comprises a screen on which are deposited luminescent substances (or phosphors) arranged in triads, each dot, referred to as a luminophor, of this triad displaying a primary color when it is excited, for example being red, green or blue. The excitation is performed by means of a beam of electrons generated by an electron gun allocated to a particular color. The three electron beams have different directions so that the three pencils of these beams which pass through one and the same aperture of the mask have different impact points on the screen, each point of impact being a part only of a luminophor, referred to as a "chromatome". The adjustment which consists in adjusting the direction of each beam so that it excites only the color for which it is provided, is referred to as the purity adjustment.
Although the purity adjustment may be correct, the color reproduction is not necessarily perfect; it is necessary, moreover, that the chromatomes of each triad should not be spaced too far apart from each other. As a matter of fact, in order that an eye receives the impression that it sees a white or uniform spot in the presence of three luminous dots of the three primary colors, these luminous dots must be close enough to each other. To secure this result, matters are organized so that the impact points of the three beams are superimposed on the screen in the absence of the mask. The setting which permits securing this result is referred to as the convergence setting.
The purity and convergence adjustments are obtained on the one hand by means of constant magnetic fields generated by one or more magnets and, on the other hand, by correct positioning of the deflector (intended to displace the electron beams for performing the scanning operation) with respect to the remainder of the tube.
The adjustment for purity and for static convergence (that is to say the convergence at the center of the screen, without scanning) is effected by means of permanent magnets. The invention relates to an adjustment of this nature.
It relates more particularly to the adjustment of the purity and of the static convergence of the tubes which comprise a magnetized ring around the neck of the tube. This ring is formed, for example, by a pliable material referred to as "plastoferrite" which is ferrite embedded in a plastics material.
In order to perform purity and static convergence adjustments, a particular number of poles are established in the magnetizable ring for example, eight, which are uniformly spread around the axis of the tube, the intensity of magnetization, that is to say the magnetic induction, of each pole being a function of the whole of the corrections which are to be performed.
The magnetization of these poles has been performed up to the present by means of coils having a particular position in a fixed support (French Pat. No. 2,399,120) or in a clamp-shaped support (French Pat. No. 2,502,386). However, since the magnetization rings are not all of precisely the same dimensions, and also the necks of the tubes are not all of identical dimensions, the intensity of the current to be supplied to each coil for magnetization of the ring--in order to obtain the purity and static convergence corrections--varies from one tube to another. The result is that the adjustment differs between one tube and another, which is troublesome as regards mass-production. Moreover, this variation renders it difficult to establish automation of the adjustment.
The present invention seeks to overcome this disadvantage.